Axial motor deficits in coordination caused by diseases, such as Parkinson disease, multiple sclerosis, and cerebellar degeneration, can contribute to falls and injuries. Although these deficits are common, relatively little is known about axial motor coordination. The proposed project will investigate how the central nervous system (CNS) coordinates axial movement by using the "sit-up" as a behavioral model. The CNS uses a complex pattern of axial motor coordination to produce the sequence of actions leading to sitting up. This sequence of actions requires precise regulation of the timing and intensity muscle activity in a large number of major muscle groups across a large part of the body. The sequential activation patterns of muscles suggests a role for proprioceptive input in the coordination of sitting up. Neurologically normal human subjects will sit up from a supine position. In sit-ups, two-thirds of the body weight in head and trunk must be lifted, and because of this weight distribution, sitting up cannot be accomplished as a simultaneous movement of the entire upper body. Instead, it must be performed as a sequence of smaller body lifts, beginning with the head and moving towards the hips. A variety of mechanical interventions will be used to explore the relationships between the patterns of muscle activity and the dynamics and kinematics of movement. Movement at different joints will be constrained, and the body weight distribution will be changed for sit-ups performed at different speeds. The vertical forces produced on the support surface will be recorded along with segmental kinematics and muscle (EMG) activity.